Field Of The Invention
The present invention relates to a low noise output buffer capable of operating at high
speeds.
Background Of The Invention
Noise in the power supplies is one of the major concerns while designing high-speed digital and analog I/O circuits. One of the major sources of supply noise is the switching of output drivers. The faster the I/O, more current it requires and that implies higher noise. This may cause functional failures on chip. So, the check on the noise has become a main concern while designing high-speed output drivers.
Further, as CMOS devices are scaled down into the deep sub-micron region, the operating frequency of an output driver is increased (e.g. frequencies over 50 MHz), which is reflected in terms of reduction in rise/fall times and pulse widths. High switching speed leads to fast rate of change of current (di/dt). A Simultaneous Switching Noise (SSN) is created when many output drivers connected to a single supply switch simultaneously in presence of chip-package interface power distribution parasitic. This SSN must be limited within the maximum allowable noise level to guarantee normal functioning of the buffers and the devices connected to same supply. Therefore, power and ground noises have to be controlled for reliable operation of the logic devices. Some of the encountered problems with false operations due to SSN are false triggering, double clocking and/or missing clocked pulses. A typical chip-package interface is shown in figure 1.
Supply and ground bounce due to SSN can be written as
Vbounce = n*L*di/dt
Where n is the number of buffers switching together, L is the cumulative inductance of trace, bonding wire and metal rail interconnects and di/dt is the rate of change of current

of an output driver flowing through supply and ground pad. As the parameter n and L (due to the limitation from packaging) are not in our hands, the only quantity that can be controlled is the current slew rate for controlling supply/ground noise.
Supply noise can be suppressed by reducing the rate of change of charging and discharging current at the load. The rate of change of charging/discharging can be monitored by controlling the signals connected to gate of output driver (i.e. GN and GP in fig. 3) and/or using appropriate sized output driver transistors. The sizes of output driver transistors are fixed due to the output impedance matching requirement with that of the characteristic impedance of the transmission line or output drive specification for driving the TTL/CMOS load.
Figure 2 shows one of the output driver final stage equivalent circuits.
Figure 3 shows a block diagram of conventional compensated CMOS output buffer. It comprises of tri-state logic 30, active slew rate control 31, compensation cell 32, output driver transistors 33 and 34 connected to output pad and load capacitor 35. The circuit shows the output buffer being compensated for slew rate at the rising edge only.
Generally, a pre-driver is used for controlling the slope of the signal connected to gate of output driver by which the slew on the rising edge can be controlled while slew on the falling edge can be controlled by sizing of output driver but in case of high speed buffers, the output current is quite high due to the low output impedance of the driver when the PAD is at VOH & VOL levels and falls abruptly when input transits. When input makes a transition from logic low to high, current at the load starts rising first and then starts falling gradually as output driver PMOS goes in linear region. Also, voltage at the PAD starts rising and reaches the required VOH value but there is an abrupt fall in the current due to change in logic at the input (from high to low) of the buffer as shown in waveform 2 of figure 4. This steep fall of current from high value to zero when PMOS goes off at the falling edge of the current creates noise due to high value of slew rate when many output buffers switch together.

Thus a circuit providing slew rate limitation at the falling edge is required. The proposed invention allows a smooth current transition as seen by the supply so that di/dt of the current flowing through supply is not big enough. An additional current bypass circuit is added to the conventional output buffer circuit that turns on as soon as either of transistors MN or MP (in fig. 6) turns off abruptly due to change in input logic. Thus the additional current in bypass circuit adds to the current flowing through output driver to make it smooth as seen by supply pad.
Objects And Summary of the Invention
To obviate the aforesaid drawbacks, the object of the invention is to control the slew rate at the falling edge of current of the CMOS output driver.
Another object of the invention is to provide a low noise output buffer capable of operating at high speeds.
To achieve the aforesaid objects the instant invention provides a low noise output buffer capable of operating at high speeds comprising of a ground/supply, a main circuit wherein a slew rate limiting means is connected to said ground/supply and said main circuit for the falling edge of the output switching signal.
Said slew rate limiting means include microelectronic transistors connected to said main circuit depending upon the requirement of the main circuit.
Said slew rate limiting means include MOS transistors connected to said main circuit depending upon the requirement of the main circuit.
Said slew rate limiting means include CMOS transistors connected to said main circuit depending upon the requirement of the main circuit. Said main circuit includes an output buffer.

A\nethod of controlling noise in output buffers capable of operating at high speeds comprising the steps of sourcing/sinking the current in the slew rate limiting means at the falling edge of the output switching signal.
Brief Description of the Accompanying Drawings
The invention will now be described with reference to the accompanying drawings.
Figure 1 is a typical chip package interface parasitics.
Figure 2 is Output Driver final stage equivalent circuit.
Figure 3 is a conventional compensated CMOS Output Driver.
Figure 4 shows the waveform in accordance with invention.
Figure 5 shows the block diagram of the instant invention.
Figure 6 is an embodiment of the instant invention.
Figure 7 shows simulation results for vdd/gnd at 55MHz at 80pf load.
DETAILED CIRCUIIT DESCRIPTION
The proposed circuit senses the change in the logic level of the input signal (waveform 1 of figure 4) and triggers on when output driver transistor goes off.
Figure 4 shows the waveforms in accordance with invention. First waveform corresponds to an input signal. The second waveform corresponds to the current profile of an output driver while charging or discharging the load. The third waveform corresponds to the current profile of the additional circuit for limiting slew rate at the falling edge of the input signal. The fourth and fifth waveforms refer to the smooth curves obtained at source/sink due to the additional circuit.

Figure 5 shows the block diagram of the instant invention. The circuit consists of a main circuit 50, additional circuit 51 for providing slew rate limitation at the falling edge and a supply 52/ground 53. The main circuit 50 is basically a circuit for which the slew rate at the falling edge is to be controlled. Both the main circuit 50 as well as the additional circuit is connected to the supply 52/ground 53 for sourcing or sinking current.
When the input signal A (waveform 1 of figure 4) is applied, the current in the main circuit 50 starts rising and follows the trajectory as shown in waveform 2 of figure 4. It can be seen from the waveform that the slew rate is controlled at the rising edge using compensation cell. As soon as the input signal (waveform 1 of figure 4) transits from high to low, to avoid abrupt current fall, the additional circuit 51 comes into play with current profile as shown in waveform 3 of figure 4. It provides an alternate path to the supply current and therefore removes the possibility of abrupt transition of current. This is further illustrated using an embodiment.
Referring to fig 6, the NIN and PIN signals are delayed version of the input signal A. When EN is logic HIGH, output driver is tri-stated, signal PIN goes low (GP goes high) and NIN goes high (GN goes low). The nodes GN and GP are controlled for slew control using pre-driver transistors 61 & 62. When output driver PMOS 63 is ON (Input high), voltage at node GP is controlled (slowly decreased) to control the slew rate at the rising edge. In the same way, signal at node GN is controlled when output driver NMOS 64 is ON. So when input changes from high to low i.e. node GP goes from low to high, output driver PMOS 63 goes off and being very high drive, current falls abruptly from high value to zero. To avoid the abrupt change in the current from supply, a PMOS 66 is turned on using signal NIN that goes low (when GP goes high) while the falling of the current from supply is controlled by charging the capacitor 68. The same process is followed when input goes low and the additional circuit NMOS 67 turns on to discharge the capacitor 68.

The^ sizes of the transistors 66 and 67 and capacitor 68 are calculated using simple analysis. In the present case this circuit is designed to operate at 55 Mhz at 80pf capacitive load with the specifications on the output driver as maximum slew rate 20 mA/ns and output impedance 50 ohm. Specified values of the VOH and VOL are 0.8*Vdd and 0.2*Vdd respectively. The peak current of the transistors 66 and 67 can be set to the same value of output current when input makes a transition i.e. when voltage at PAD crosses VOH for 66 and VOL for 67. The value of the capacitor 68 can be set for the desired falling slew rate of the supply current.
SIMULATION RESULTS
Figure 7 shows the three graphs plotted to show the noise reduction using the instant invention over the prior art.
First diagram shows the pulse input and output of the output driver.
Second diagram shows the noise in the 2.1V supply with and without using additional circuit. It can be seen that there is not much of a difference in the noise at the rising edge but the noise at falling edge is drastically reduced.
Third diagram shows the ground noise with and without additional circuit. The difference in the ground noise at the falling edge of the current can be seen. The results have also been tabulated in the table no. 1.

(TABLE REMOVED)Tablel: Supply/ground noise comparison
When the additional circuit of the proposed invention is used, at lower frequencies output driver current has enough time to come down to low value. This creates a small spike of current due to additional circuit which adds a bounce to supply, but it is insured that the bounce created is not going to effect the normal operation as the level of bounce created is lower than the bounce created by the rising edge slew of the current.
Looking at the results, it can be concluded that the above-mentioned invention for controlling slew rate is very effective if the output driver is used at a predetermined frequency. This circuit has been designed for worst case where the effect of noise on the driver is maximum. But the proposed circuit can be compensated using the digital codes from compensation cell to make if even more effective at the slow corners. This is a novel method/'apparatus to increase operating frequency without increase in supply/ground bounce.

We Claims
1. A low noise output buffer capable of operating at high speeds comprising of a ground/supply, a main circuit wherein a slew rate limiting means is connected to said ground/supply and said main circuit for the falling edge of the output switching signal.
2. The low noise output buffer as claimed in claim 1 wherein said slew rate limiting means include microelectronic transistors connected to said main circuit depending upon the requirement of the main circuit.
3. The low noise output buffer as claimed in claim 1 wherein said slew rate limiting means include MOS transistors connected to said main circuit depending upon the requirement of the main circuit.
4. The low noise output buffer as claimed in claim 1 wherein said slew rate limiting means include CMOS transistors connected to said main circuit depending upon the requirement of the main circuit.
5. The low noise output buffer as claimed in claim 1 wherein said main circuit includes an output buffer.
6. A method of controlling noise in output buffers comprising the steps of sourcing / sinking the current in the slew rate limiting means at the falling edge of the output switching signal.
7. A low noise output buffer capable of operating at high speeds substantially as herein described with reference to and as illustrated in the accompanying drawings.
8. A method of controlling noise in output buffers substantially as herein described with reference to and as illustrated in the accompanying drawings.